If a page doesn't provide its own icon via propertyTabPage:icon, a default
icon may be used instead for contexts where having an icon is necessary.
classTabBar will use default icon for pinned tabs in case the page is not
loading, doesn't have an icon and an indicator. Default icon is never used
for tabs that aren't pinned.
classTabOverview will use default icon for pages with missing thumbnails.
By default, the adw-tab-icon-missing-symbolic icon is used.
Sets the default page icon for self.
If a page doesn't provide its own icon via propertyTabPage:icon, a default icon may be used instead for contexts where having an icon is necessary.
classTabBar will use default icon for pinned tabs in case the page is not loading, doesn't have an icon and an indicator. Default icon is never used for tabs that aren't pinned.
classTabOverview will use default icon for pages with missing thumbnails.
By default, the adw-tab-icon-missing-symbolic icon is used.